DESCRIPTION The purpose of this four-year study, Community-Based Research on Drug Use Networks is to investigate the drug use and sexual networks of chronic out of treatment drug users and the relationship between drug-use and sexual network structures and HIV infection and sexually transmitted diseases. The study is based upon the social network assumption that how groups of people interact is related to the transmission of HIV and other sexually transmitted diseases. Therefore it is assumed that how network structures change over time will be related to varying rates of transmission among groups of people. The study will be conducted using a longitudinal panel design. The sample of networks to be investigated will be drawn from a pool of chronic out of treatment drug users residing in three targeted communities in Houston Texas. The study will recruit the drug use and sexual networks of 125 focal respondents selected to participate in the study. Outreach workers will assist focal respondents in recruiting their drug use and sexual contacts to participate. All drug use and sexual contacts during the precious 30 days will be recruited. Data will be collected from participants at intake and 12 months after intake. Approximately 600 respondents are anticipated in each wave of the study. Self-report and biological data will be collected. Data will be analyzed using a variety of statistical methodologies. At minimum the proposed research project will address the following research questions: 1. What are the different types of drug use and sexual network structures of chronic, out-of-treatment drug users? 2. Which network roles or positions can be identified in drug use and sexual networks? 3. How are these roles or positions related to the transmission of HIV or syphilis within and between such networks? 4. How do network structures and roles differ by demographic, relationship, behavioral, and disease variables-particularly HIV and syphilis incidence? 5. How do network structures and roles differ by gender and ethnic group? 6. How do network structures and roles change over time? How are these changes related to HIV risk?